1. Field of the Invention
The present invention relates to a sheet ejecting or discharging apparatus having a curl correcting means, and more particularly, it relates to a sheet discharging apparatus used with an electrophotographic system such as a copying machine, laser beam printer and the like. More specifically, the present invention relates to a sheet discharging apparatus wherein the curl generated in a sheet at a fixing station is corrected by a curl correcting mechanism and then the sheet is discharged or ejected in a face-up or face-down mode.
2. Related Background Art
As shown in FIG. 7, a conventional fixing device includes a fixing roller 30 comprising a metallic pipe for fusing and fixing a toner image on a transfer sheet by utilizing heat generated by means of a halogen heater 31, and an elastic pressure roller 32 urged against the fixing roller 30 by means of a pressure spring 33. However, in such a fixing device, when the image formation is to be started with the printer completely cooled, it is necessary that the halogen heater 31 is energized for a long time to perform a warming-up operation for adequately heating the fixing roller 30. Consequently, it takes a long time the image formation permissible condition to be established after turning on of the power source, and a large amount of electric power is consumed.
Thus, in place of the fixing roller 30 and the halogen heater 31, as shown in FIG. 8, there has been proposed a fixing system comprising a fixedly supported flat plate-shaped ceramic heater 35, a heat-resisting film 36 shiftable while contacting with the heater 35, and a pressure roller 37 for urging a transfer sheet P against the heater 35 via the film 36 (refer to Japanese Patent Laid-open No. 63-313182). More specifically, as shown in FIG. 9, the heat-resisting film 36 is held for rotation around a guide member 39. The elastic pressure roller 37 is disposed in confronting relation to the ceramic heater 35 and is urged against the heater with the interposition of the film 36 therebetween by means of a compression spring 40. In such a fixing system, even if the whole fixing system is completely cooled, since the transfer sheet P is urged against the heater 35 with the interposition of the thin film 36 having a thickness of several tenth microns, the warming-up time can be considerably reduced and thus, the consumption of the electric power can also be reduced greatly.
Incidentally, in the fixing system of FIG. 8, as shown in FIG. 9, immediately before the film 36 enters into a fixing nip between the rollers, the film is slightly tensioned by means of a projection 41a integrally formed on a heater holder 41 in the proximity of the heater 35 at an upstream side thereof, so that wrinkles and irregularities in the film are removed to form a stable flat nip portion as indicated by D. However, as shown in FIG. 8, with this arrangement, since the pressure at a central portion B in the nip is slightly greater than those at upstream and donwstream portions A, C, the transfer sheet P is curved to form a downwardly directed concave portion.
If the transfer sheet with the downwardly directed concave portion (generated at the nip portion) is heated by the heater 35 and then is subjected to natural cooling, a great downwardly directed concave curl is generated in the transfer sheet, as shown by the portion D in FIG. 9. To avoid this, as shown in FIG. 10, before the transfer sheet P is subjected to the natural cooling, the transfer sheet P with the downwardly directed concave portion (generated at the fixing nip portion D) is forcibly curved upwardly by providing a sheet discharge guide 42 extending leftwardly and upwardly from the downstream side of the pressure roller 37, as shown by a portion E in FIG. 9. Then, by cooling the transfer sheet P naturally, the curl in the sheet is surpressed.
The transfer sheet P that the curl is corrected as mentioned above is suited for an electrophotographic system having a sheet discharging apparatus for discharging a sheet in a face-up manner. However, when the sheet is discharged in a face-down manner from the fixing device, there arises a problem that the sheet is again curled upwardly at an inverting portion 43 (portion F) (refer to FIG. 11). To avoid this, a radius of curvature of the inverting portion 43 shown in FIG. 10 may be increased, or the transfer sheet P may be forcibly cooled by using a fan and the like, or a distance between the fixing device and the inverting portion 43 may be increased so as to permit the adequate cooling of the sheet. However, these techniques will make the compactness and cost-down of the laser beam printer and the like difficult.